Palm oil is fractionated in one- or two-staged by solvent fractionation, detergent fractionation, dry fractionation and the like. Generally, in the case of two-stage fractionation, an olein fraction (lower melting point fraction) obtained by the first stage fractionation is separated into a fraction called as super olein and a middle melting point fraction (PMF).
Among the above mentioned fractionation methods of palm oil, a method which is most generally employed today is dry fractionation wherein treatment can be carried out inexpensively. However, the fractionation method has such a defect that the precision of fractionation is lower in comparison with other fractionation methods because a liquid portion which is taken into crystals during crystal growing process is larger. For example, the yield of olein obtained by one-stage fractionation of palm oil is not less than 70% in the case of solvent fractionation or detergent fractionation method, while the yield is often lower than 65% in the case of dry fractionation method [J.A.O.C.S., 62 (2) 210-219 (1985)].
This tendency is increased as the amount of a crystal part formed becomes larger and, therefore, in the above-mentioned general two-stage fractionation, the precision of fractionation becomes more important in the second stage than that in the first stage. In particular, it is desired to obtain PMF having a lower iodine value (hereinafter abbreviated as I.V.) in view of production of PMF to be used for hard butter. However, in order to reduce the I.V. to lower than 50, there is required other means for increasing the precision of fractionation, for example, means for increasing compression pressure applied to solid-liquid separation or the like.
In addition, since various impurities are contained in crude palm oil, which causes blinding when loaded on a crystallizing apparatus or a filter and varies the quality of the desired product, generally, refining steps such as degumming, decolorization and physical deacidification (RBD: Refined Bleached Deodorized), or neutralization, decolorization and physical deacidification or deodorization (NBD: Neutralized Bleached Deodorized) are carried out before dry fractionation. And, in an apparatus used therefor, dry fractionation means is connected to physical refining means.